Electrophotographic marking is a well known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a copy of the desired image. The surface of the photoreceptor is then cleaned and recharged for the production of another image.
The foregoing broadly describes a prototypical black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner used to make the composite color image. The various color toners can then be transferred onto a substrate in a superimposed registration such that a desired composite color image results. That composite color image can then be fused to make a permanent image.
One way of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is typically comprised of a laser light source (or sources) and a rotating polygon having a plurality of mirrored facets. The light source radiates a laser beam onto the polygon facets and the facets then reflect the beam onto the photoreceptor, producing a light spot. As the polygon rotates the spot traces lines, referred to as scan lines, on the photoreceptor. By moving the photoreceptor as the polygon rotates the surface of the photoreceptor is raster scanned by the spot. During scanning, the laser beam is modulated with image information so as to produce a predetermined latent image on the photoreceptor. For color printing, by repeating the raster scanning to create a latent image for each color of toner a predetermined color image is produced.
While raster output scanning is successful, it has problems. For example, in color electrophotographic printing it is very important that the various color images are properly registered. By registration it is meant that the latent images are created at predetermined locations on the photoreceptor such that when the various latent images are developed and transferred onto a substrate that the proper final composite image results. Specifically important to the present invention is the fact that the motion of the photoreceptor is not perfect. Vibration, motor backlash, gear train interactions, mechanical imbalances, friction, among other factors, cause the instantaneous position of the photoreceptor to be less than ideal. Therefore, without compensation, the scan lines of the various images are not fully registered.
Color print testing performed at Xerox has proven that motion quality problems result in a color defect that is referred to herein as motion quality induced color banding. Color banding itself is a term given to any color image to color image misregistration. Because motion quality induced color banding is detrimental to print quality a technique of reducing or eliminating motion quality induced color banding would be beneficial.